The present invention relates to a switching power supply device which is preferably used as a so-called A.C.-D.C. convertor, D.C.xe2x80x94D.C. convertor and so forth.
There are switching power supply devices widely used for various apparatuses such as mobile electronic apparatuses. The devices switch a D.C., which is either obtained by rectifying/smoothing commercial A.C. or supplied from a battery, by applying high frequency around several hundred kHz so that the supplied voltage is effectively converted to a target value by means of a small transformer.
A typical example of this type of power supply device which has been widely used is a pulse-width modulation (PWM) type switching power supply device, which is a switching power supply device arranged such that in accordance with a result of detecting an output voltage on the secondary winding side by a voltage detection circuit, a control circuit controls a width of a switching pulse of a main switching element so that a target output voltage on the secondary winding side can be obtained.
Another example of the power supply device having been widely used is a ringing choke convertor (RCC) type switching power supply device, which is a power supply device arranged such that exciting energy accumulated in a transformer during a main switching element being turned on is transmitted to a circuit in the secondary winding side during the element being turned off, and after the transmission being finished the main switching element is turned on again by feeding a ringing pulse generated in a control winding of the transformer back to a control terminal of the element via a D.C. cut capacitor.
FIG. 8 is a block diagram showing an electrical arrangement of a typical conventional switching power supply device 1. This switching power supply device 1 is an RCC-type switching power supply device in which a D.C., obtained by rectifying a commercial A.C. 3 by a main power supply circuit composed of a diode bridge 2 and a smoothing capacitor c1, is supplied to a circuit between a main power supply line 4 of a high-level side and a main power supply line 5 of a low-level side. The circuit between the main power supply lines 4 and 5 is arranged as a primary winding n1 of a transformer n and a main switching element q are connected in series. The main switching element q is, for instance, a bipolar transistor or a field-effect transistor. Incidentally, in this example shown in FIG. 8, the field-effect transistor is adopted as the main switching element. The main switching element q is turned on or off by a control circuit 6.
When the main switching element q is turned on, exciting energy is accumulated in the primary winding n1. Then the energy is induced to the side of a secondary winding n2 during the element q is turned off, and after being smoothed in a diode d1 and a smoothing capacitor c2, the energy is sent from output terminals 7 and 8 to an unillustrated load.
An output voltage between the output terminals 7 and 8 is divided by voltage dividing resistors r1 and r2, and then the divided voltage is sent to one input of a comparator circuit 9, while to the other input of the comparator circuit 9, a reference voltage from a reference voltage source 10 is supplied. The value of the divided output voltage is compared with the reference voltage, and the result of the comparison is fed back to the control circuit 6 via a photodiode d2 of a photocoupler pc1 and a phototransistor tr1. The control circuit 6 transmits a drive signal to the main switching element q on the basis of: a D.C. voltage provided via a starting resistor r3; an induction voltage transmitted from an auxiliary winding n3 of the transformer n; and output voltage information transmitted from the comparator circuit 9 via the photocoupler pc1, so that the circuit 6 controls the switching of the main switching element q to stabilize the output voltage of the switching power supply device 1.
In the meantime, the switching power supply device 1 is provided with members for conducting the switching intermittently, namely a photocoupler pc2, a logic circuit 11, comparators 12 and 13, the above-mentioned reference source 10, and voltage dividing resistors r4, r5, and r6, for lowering power consumption of the device 1 when an apparatus supplied power from the device 1 is in standby. In this arrangement, the control circuit 6 makes the main switching element q intermittently switch on or off at the time when a standby signal is supplied from a control input terminal 14.
That is to say, the comparator 12 monitors the output voltage of the switching power supply device 1 so as to send a stop instruction to the control circuit 6 via the logic circuit 11 and the photocoupler pc2 when a voltage value of the high-level side, divided by the voltage dividing resistors r4, r5, and r6, is not less than the reference voltage of the reference voltage source 10, so that the comparator 12 stops the operation of the switching power supply device 1. The comperator 13 also monitors the output voltage of the switching power supply device 1 so as to send a start instruction to the control circuit 6 via the logic circuit 11 and the photocoupler pc2 when a voltage value of the low-level side, divided by the voltage dividing resistors r4, r5, and r6, is gradually decreased due to the stop of the switching operation so as to be not more than the reference voltage, so that the comparator 13 starts the operation of the switching power supply device 1. The output voltage starts to increase again as the operation is resumed, and when the divided voltage value of the high-level side becomes not less than the reference voltage, the operation stops again, so that this sequence is repeated.
The conventional techniques have a problem such that a circuit arrangement is complicated. That is to say, apart from the comparator circuit 9 for monitoring the output voltage to stabilize the same, it is necessary to additionally provide the comparators 12 and 13 which are voltage monitoring means. Moreover, in accordance with the voltage information detected in the secondary winding side of the transformer n, the control circuit 6 in the primary winding side makes the switching power supply device operate intermittently. In this case the primary winding side cannot be directly connected to the secondary winding side since it interrupts the input being insulated from the output, so that these two have to be connected via the photocoupler pc2. Furthermore, the device has to receive the standby signal from the apparatus to which the power is supplied so that the device has to be connected to the apparatus using a control line as well as the power supply line, and moreover the apparatus has to be arranged as being capable of sending the control signal, when the device is adopted as an A.C. adapter.
The present invention aims at providing a switching power supply device with low power consumption when an apparatus to which the device supplies the power is in standby, with a simple arrangement.
To achieve the above-mentioned aim, the switching power supply device of the present invention, provided with control means for controlling switching of a main switching element that switches on or off a supplied D.C. voltage in accordance with output voltage information fed back from a secondary winding side so that an output voltage is stabilized to be a target voltage, includes:
fluctuation detection means, for detecting a fluctuation of controlling of the control means and transmitting a detected output in accordance with nonlinear output characteristics, only under a light load; and
superposition means, for attracting a fluctuation in the output from the fluctuation detection means and superposing the fluctuation on the output voltage information,
wherein the control means judges that the device operates under the light load in accordance with the fluctuation, which has been fed back.
Thus this enables the control means to judge that the device operates under the light load in accordance with the fluctuation so that it is unnecessary to supply a signal indicating the weight of the load from the outside, and hence it is possible to provide the switching power supply device with low power consumption when an apparatus to which the device supplies the power is in standby, with a simple arrangement.
The switching power supply device of the present invention may be arranged such that: the superposition means is a differentiating circuit including a capacitor for removing a D.C. component and voltage dividing resistors for dividing an output voltage so as to produce the output voltage information; the output from the fluctuation detection means is supplied to a node between the voltage dividing resistors via the capacitor; and the output voltage information is produced by comparing a voltage at the node with a reference voltage by means of a comparator circuit.
According to this arrangement, the differentiating circuit can extract an A.C.-like component in the control fluctuation and superpose the component on the output voltage information. Thus it is possible to adopt a simple circuit, in which the existing voltage dividing resistors are provided with the capacitor for removing a D.C. component, as the superposition means.
The switching power supply device of the present invention may be arranged such that the fluctuation detection means includes: a series circuit, provided in parallel with a secondary winding of a transformer and composed of a diode and a capacitor, which is charged by an induction voltage having one polarity so as to supply the D.C. voltage in accordance with electric charges of the capacitor; and a load resistor for discharging electric charges.
According to this arrangement, it is possible to transform the output into a D.C. voltage level in accordance with the nonlinear characteristic so that the D.C. voltage level indicates a high frequency voltage of the control fluctuation being detected and rectified.
Thus the fluctuation detection means can obtain the nonlinear characteristic with a simple arrangement so that it is possible to: certainly remove a fluctuation component under the heavy load; detect only the fluctuation component under the light load; and feed the same back.
The switching power supply device of the present invention may be arranged such that the fluctuation detection means further includes a current limitation resistor, which is provided in series with respect to the diode, for limiting a charge current of the capacitor.
According to this arrangement, it is possible to control the amount (degree) of superposition of the output voltage from the fluctuation detection circuit on the output voltage information. On this account, it is possible to adjust the critical point of the nonlinear characteristic.
Thus it is possible to set the level of the output voltage at the moment of shifting from the state of continuous switching operation to the state of intermittently conducted switching operation.
The switching power supply device of the present invention may be arranged such that the fluctuation detection means further includes a Zener diode provided in parallel with the capacitor.
According to this arrangement, provided that the Zener diode has a Zener voltage at the critical level of the nonlinear characteristic, a voltage higher than the Zener voltage is eliminated so that it is possible to make the nonlinear characteristic close to an optimum characteristic.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.